The present invention relates to the field of electrochemical oxygen sensors which provide a quantitative indication of the amount of oxygen in an environment.
Electrochemical oxygen sensors have been known in the art for a long time, and include the so-called galvanometric cells and the so-called polarographic cells. Galvanometric cells are, in effect, miniature fuel cells which generate an electric current when oxygen comes into contact with the electrodes. Polarographic cells require an external voltage source. The cell of the present invention is of the galvanometric type, but the structure disclosed herein could also be applied to cells of the polarographic type.
Examples of electrochemical cells used as oxygen sensors are shown in U.S. Pat. Nos. 3,429,796 and 3,767,552. Both of these patents show electrochemical cells, of the galvanometric type, having a cathode and anode, disposed within a housing, an electrolyte contacting the cathode and anode, and a gas-permeable liquid-impermeable membrane which is spread over the cathode. Oxygen from outside the cell can enter the cell through the membrane, but the liquid electrolyte cannot pass through the membrane in the other direction.
The cathodes in the cells described in the above-mentioned patents provide a site for reduction of oxygen into hydroxyl ions. The hydroxyl ions react with the anode to form an oxide of the anode, while also releasing electrons. These oxidation and reduction reactions together produce an electric current when a load is connected across the electrodes.
While the operation of an oxygen sensor cell is, in theory, quite simple, many practical problems arise in the design of such cells. For example, it is necessary that the cell be completely sealed from the outside environment. If the electrolyte leaks out, the effectiveness of the cell will be gradually reduced, and the overall life of the cell will be shortened. It is also important that leakage not occur from the outside into the cell.
Electrochemical oxygen sensors should also show linearity, i.e. the current produced should in fact be proportional to the amount of oxygen in the outside environment. Moreover, the cell must be capable of generating a current as soon as even a small amount of oxygen enters the cell.
Another problem encountered with electrochemical oxygen sensors is breakage of the components during assembly, shipment, or use. In the sensors of the prior art, it has been customary to connect the electrodes to their terminals by spot welding a wire to the electrodes and the terminals. This type of connection sometimes breaks during handling, or when the components of the cell are being assembled.
The present invention comprises an improvement in an electrochemical cell structure, wherein the cell is well-protected against leakage to and from the outside, and the gas diffusion membrane is protected from damage caused by sharp objects. The cell is easy to assemble, and the connection between the cathode and its terminal is such as to minimize the likelihood of breakage. Embodiments of the invention are described for use in both normal and hyperbaric environments.